This invention relates to the production of articles by injection or blow molding, and more particularly, to a method for using stereolithography to produce molds for injection or blow molding apparatus.
Prototype containers have been produced by having a craftsman carve a solid pattern out of a section of wood or plastic. This was very common practice ten to fifteen years ago and is still practiced today. The advantage of this method of producing prototype containers is primarily that the process is relatively inexpensive. A disadvantage of this process is that the models frequently are different than what the designer had in mind. In addition, if more than one model is requested, there is a high probability that the models produced will not be identical. It is also difficult to convey end use properties of a plastic container with a wood model.
An improvement over the hand production of models included the use of computer aided design (CAD) models to generate the control signals for computer numerical control (CNC) paths for a cutting tool. Computer numerical control machining requires a CAD model to determine the paths for the cutting tool to follow when cutting actual steel tooling. Using this technology, it is possible to cut a single cavity, prototype tool from aluminum or other metal. Disadvantages of this method are that it is costly, typically $15,000 to produce a 2 liter beverage container, and usually requires 8-10 weeks in lead time. An advantage of this method is the metal tool has a longer lifetime than other prototype tools. However, this is not a great consideration for a prototype tool because a prototype tool will be used for limited production, typically producing only 50 to 100 parts.
The introduction of epoxy molds has provided a way in which a mold can be produced relatively quickly for less expense than cutting metal molds, yet still using the CAD design. Production of the epoxy mold begins with a pattern or model of the outside contour of the container, produced commonly using stereolithography. The two halves of the pattern or model of the container are then set into an aluminum shell and a castable epoxy is poured around the pattern or model. When the stereolithography pattern is removed, typically by melting the pattern, the void left in the epoxy mold has the shape of the container. An advantage of this process is that the epoxy mold represents the geometry and contours of the surface of the container fairly accurately, using the CAD generated solid pattern or model built on the stereolithography machine. Also, the process is fairly quick, taking approximately two weeks from initiation of the design work to the completion of the epoxy mold. A disadvantage associated with the use of epoxy molds is that the durability of epoxy molds is very limited in use. In some cases, the epoxy molds failed after only one or two production cycles. However, producing epoxy molds is considerably less expensive than manufacturing a metal tool with the cost of epoxy tools typically being in the range of $3500 to $4000.